The background of the invention will be set forth in two parts.
1. Field of the Invention
This invention relates to delay lines and more particularly to bulk acoustic wave delay lines.
2. Description of the Prior Art
The usefulness of propagating elastic wave energy in solids has been known for many years and is generally known as microwave acoustic technology. Utilizing this technology, such devices which store and delay signals have been developed to a relatively high degree. Many texts are presently available which thoroughly describe the history and advancements of this art, such as, for example, "Ultrasonic Methods I Solid State Physics" by Rohn Truell, Charles Elbaum and Bruce B. Chick, Academic Press, 1969.
Probably of greatest interest in the field of bulk wave devices has been bulk wave delay lines. A bulk acoustic wave delay line typically consists of thin film piezoelectric transducers deposited on the parallel faces of a crystal, such as quartz for example, which is capable of supporting propagating bulk acoustic wave energy. The effective size of the transducer is determined by the metal top electrode, which, in consideration of capacitance and diffraction loss, is generally chosen to be 0.002 to 0.040 inch in diameter. The reception of the beam at the receiving end requires registration of the receiving transducer metal top electrode to a fraction of this diameter. Of importance is the fact that the location of the beam at the output or receiving end depends not only on the input transducer location, but also on the crystallographic orientation and end face angles. Thus, it has been a significant problem to achieve adequate electrode location in a bulk wave device. For beam diameters of 0.002 inch, it has been virtually impossible.
In the past, such techniques as very high tolerance X-ray crystal orientation, crystal flat orientation, and lithography registration tooling have been proposed and attempted in order to overcome this problem. Another prior art technique has been to fabricate a single transducer on one end of a crystal, and then attempt to find the acoustic beam at the other end using acousto-optic Bragg scattering techniques, followed by the disposing of a transducer or multiplicity of transducers in the approximate required location.